1. Field of the Invention
This invention relates to a method for the continuous preparation of isobutylene substantially free from tertiary butyl formate and peroxides from a tertiary butyl alcohol (TBA) feedstock contaminated with tertiary butyl formate, peroxides and ketones wherein the contaminated tertiary butyl alcohol feedstock is continuously charged to a reactor and contacted therein with a bed of catalyst consisting essentially of sulfuric acid impregnated alumina under reaction conditions including a temperature of about 150.degree. to about 400.degree. C., a pressure of about 0 to about 3000 psig. and a feed rate of about 0.5 to about 10 g of feed per feedstock per hour per cc of catalyst to form a reaction product substantially completely free from tertiary butyl formate.
2. Prior Art
It is known to react isobutane with oxygen, either thermally or catalytically, to form a peroxidation reaction product wherein the principal peroxide that is formed is tertiary butyl hydroperoxide. It is also known to thermally or catalytically decompose the tertiary butyl hydroperoxide to form tertiary butyl alcohol.
Thus, it is known that isobutane can be oxidized with molecular oxygen to form a corresponding tertiary butyl hydroperoxide and by-products and that the reaction can be promoted, for example, with an oxidation catalyst (see Johnson U.S. Pat. No. 3,825,605 and Worrell U.S. Pat. No. 4,296,263).
A process for the manufacture of substituted epoxides from alpha olefins such as propylene is disclosed in Kollar U.S. Pat. No. 3,351,653 which teaches that an organic peroxide compound such as tertiary butyl hydroperoxide can be reacted with an olefinically unsaturated compound such as propylene in the presence of a soluble molybdenum catalyst. The products of the reaction are propylene oxide and tertiary butyl alcohol.
Thus, tertiary butyl alcohol can be prepared either by the direct thermal or catalytic reduction of tertiary butyl hydroperoxide to tertiary butyl alcohol or by the catalytic reaction of propylene with tertiary butyl hydroperoxide to provide propylene oxide and tertiary butyl alcohol.
When isobutane is reacted with oxygen, a wide variety of oxidation by-products are also formed in small amounts including, for example, methyl formate, acetone, 2-butanone, isobutylene oxide, isobutyraldehyde, methanol, methyl tertiary butyl peroxide, isopropyl alcohol, tertiary butyl alcohol, ditertiary butyl peroxide, tertiary isopropyl peroxide and tertiary butyl formate.
These oxidation by-products will remain with the tertiary butyl alcohol product that is formed by the direct or indirect decomposition of the tertiary butyl hydroperoxide.
Sanderson et al. U.S. Pat. No. 4,873,380 discloses, for example, that the peroxide-type contaminants can be removed from tertiary butyl alcohol by bringing the contaminated tertiary butyl alcohol into contact with a nickel, copper, chromium and barium catalyst. The resultant tertiary butyl alcohol product is substantially free from the other peroxide contaminants.
Processes for the preparation of tertiary butyl alcohol from tertiary butyl hydroperoxide are disclosed, for example, in a series of Sanderson et al. patents (U.S. Pat. Nos. 4,910,349; 4,912,266; 4,912,267; 4,922,033; 4,922,034; 4,922,035; 4,992,036; 4,992,602; and 5,025,113).
It is also known that tertiary butyl alcohol can be dehydrated to form isobutylene. For example, Grane et al. U.S. Pat. No. 3,665,048 discloses a process for obtaining essentially pure isobutylene by the controlled dehydration of tertiary butyl alcohol in the presence of an aluminum oxide catalyst.
European Patent Application 0255948, filed May 8, 1987, by Mitsubishi Rayon Company, Ltd., in the names of Kazutaka Inoue et al. discloses a process for the production of isobutylene by the dehydration of tertiary butyl alcohol in gas phase over a fixed bed-type of silica alumina catalyst in the presence of added nonreactive gas to inhibit isobutylene polymer formation.
The dehydration of tertiary butyl alcohol to isobutylene is also disclosed in an article by Heath et al. ("Acid Resin Catalysis: The Dehydration of t-butyl Alcohol", AICHE Journal (Vol. 18, No. 2, March 1972, pp. 321-326)).
The prior art references directed to the dehydration of tertiary butyl alcohol to form isobutylene are not directed to the problem that is involved by the presence of tertiary butyl formate as a contaminant which is the case when the tertiary butyl alcohol is formed directly or indirectly from tertiary butyl hydroperoxide. Thus, the tertiary butyl formate will normally be present in the tertiary butyl alcohol feedstock and will also be present in the isobutylene reaction product as a contaminant. The prior art processes use essentially pure tertiary butyl alcohol as a product.
If the isobutylene is reacted with methanol to provide methyl tertiary butyl ether, a motor fuel additive, the tertiary butyl formate and peroxides will be present in the methyl tertiary butyl alcohol motor fuel additive and would be very deleterious insofar as the octane enhancing qualities of the methyl tertiary butyl ether are concerned. Therefore, removal of the tertiary butyl formate and peroxides is very important when tertiary butyl alcohol is to be dehydrated to form isobutylene for use in the preparation of methyl tertiary butyl ether.